Field of the Invention
The present disclosure relates to the field of fluid control components. More particularly, the present disclosure relates to the field of hydraulic valves and regulators used to control fluid operated devices, such as other valves and components including oilfield well drilling and production equipment, such as surface and subsea blowout preventers.
Description of the Related Art
Hydraulic valves are used to control the opening and closing of hydraulically operated oilfield well drilling and production equipment such as additional valves or blowout preventers. Regulators are used to control the pressure in a hydraulic circuit to ameliorate pressure spikes which can occur when hydraulic valves in the circuit are opened or closed. Variable orifices are used to selectively pass pressure and fluid therethrough at levels between full fluid flow and pressure and no fluid flow and pressure, and thus regulate the fluid pressure downstream therefrom. Pressure regulators are used to maintain a desired pressure in the hydraulic control circuit. The hydraulic control circuit components are commonly provided with redundancy, to ensure that when required to, for example, operate a blowout preventer to close off a well bore being drilled, the hydraulic control circuit will deliver the required fluid in the required time with sufficient volume and pressure to close the blowout preventer.
One recurring limitation in hydraulic valves and regulators, which use pressurized fluid or an electromechanical actuator to cause at least one valve component to move with respect to another valve component, is stiction, which is the static friction present between two stationary surfaces in contact with one another. Typically, the force needed to overcome stiction to allow one surface to move with respect to the other is greater than the force needed to cause two surfaces in contact with each other to continue moving with respect to each other once movement therebetween has started. As a result, it is known in the art that up to 20% of the total force, and thus of the total energy, supplied to a hydraulic valve can be taken up to overcome stiction. In the regulator, where dead bands on the order of 20 to 30% are known to occur in current designs, hunting, or oscillating around the outlet pressure setpoint, is a continuing issue affecting the operation of the hydraulic circuit. Pressure oscillations in the line or conduit opened by the valve when a valve is opened on the order of 1400 psi decreases and 600 psi increases are known to occur.
An additional issue present in hydraulic circuit control components is reliability of the hydraulic control component due to wear and corrosion of the components, caused by the exposure of the components to erosive and corrosive hydraulic operating fluids, and by relative movement of the components with respect to each other. Corrosion and erosion of the relatively moving parts can generate debris tending to cause these components to become locked, or move erratically, with respect to each other, and corrosion, erosion and wear can cause sliding interface between components to leak, reducing the effectiveness and reliability of the hydraulic control circuit component. In either case, the hydraulic circuit component will require repair or replacement, which in a subsea environment is expensive where servicing of the components often requires the use of a submersible robot to remove or service a hydraulic circuit component. To prevent the hydraulic control circuit from becoming non-functional as a result of a failure of a hydraulic control component, and to reduce the number of service operation periods in which a submersible robot is used to replace components, subsea control systems often have even greater redundancy requiring even more redundant hydraulic circuits and attendant components, leading to even greater cost.
To help reduce wear, the hydraulic control circuit components which include sliding contact surfaces have been made from, or coated with, carbide materials. However relatively high stiction occurs between two closely fitted, but movable with respect to each other, carbide surfaces. As a result, to operate these hydraulic circuits, fluid maintained at relatively high pressures is required. A substantial amount of energy is used to pressurize the fluid, and large accumulators are needed to store the fluid under the high pressure. Because of the need for redundant components systems, these costs are magnified where stiction is a large factor in the operational energy needed to operate the valve.
Additionally, carbide based components are brittle in comparison to stainless steel components, and for example, where two such parts of a component must be moved into sealing engagement, slower component velocities resulting in lower engagement forces are used to ensure the components does not fracture, crack or create particles of the component which can become lodged between moving surfaces and lock the moving parts in place. As a result, slower valve operation than optimal results.